Hydrogeology of a hazardous-waste disposal site near Brentwood, Williamson County, Tennessee

U.S. Geological Survey, Water Resources Investigations Report 89-4144

Abstract

Approximately 44,000 gallons of industrial solvent wastes were disposed in pits on a farm near Brentwood, Tennessee, in 1978. Contaminants associated with these wastes were reported in the soil and shallow ground water on the site in 1985. In order to enable the State to evaluate possible remedial-action alternatives, an 18-month study was conducted to &fine the hydrogeologic setting of the disposal site and surrounding areas.

The area is underlain by four hydrogeologic units: (1) an upper aquifer consisting of saturated regolith, Bigby Limestone, Cannon Limestone, and weathered Hermitage Formation, (2) the Hermitage confining unit, (3) a lower aquifer consisting of the Carters Limestone, and (4) the Lebanon confining unit. Wells tapping these aquifers generally are low yielding (less than 1 gallon per minute), although locally yields may be as much as 80 gallons per minute. Aquifer test results indicate that the lower aquifer is anisotropic, and transmissivity of this aquifer is greatest in a northwest-southeast direction.

Recharge to the ground-water system is primarily from precipitation, and estimates of average annual recharge rates range from 6 to I.5 inches per year. Discharge from the ground-water system is primarily to the Little Harpeth River and its tributaries. Ground-water flow at the disposal site is mainly to a small topographic depression that drains the site.

Geochemical data indicate several distinct water types related to four zones of circulating ground water: (1) a shallow zone of rapid circulation; (2) a deeper zone (greater than 100 feet) of rapid circulation; (3) a shallow zone of slow moving circulation, and (4) a deeper zone of slow circulation. Both the geochemical data and a numerical ground-water flow model of the study area support the concept of two aquifers separated by a low permeability confining unit. Results of the numerical model indicate that most of the ground-water flow is in the upper aquifer, and that less than 1 percent of the recharge to the upper aquifer flows down to the lower aquifer.